A known shift-by-wire system includes a rotary actuator as a driving device for switching a shift range of an automatic transmission device. For example, JP-A-2009-177982 discloses a rotary actuator including an electric motor and a reduction gear. In the configuration disclosed by JP-A-2009-177982, the electric motor and the reduction gear are arranged to be adjacent to each other in the axial direction to reduce the total thickness of the rotary actuator and to enhance output torque of the rotary actuator.
The reduction gear of the rotary actuator includes an eccentric portion, which is eccentric relative to an output shaft. The eccentric portion supports a sun gear. The sun gear includes outer teeth at that outer circumferential periphery. The outer teeth of the sun gear meshes with a ring gear. The sun gear is configured to perform a rotary motion centering on the eccentric portion simultaneously with an orbital motion along the inner gear of the ring gear. The sun gear includes a projected portion projected from the sun gear. The output shaft includes a flange portion having a fitting hole. The projected portion of the sun gear is loosely fitted to the fitting hole of the output shaft thereby to enable transmission of rotation of the sun gear to the output shaft.
The eccentric portion rotatably supports the sun gear via a bearing. The sun gear is applied with a moment of force in a direction different from the rotative direction due to engagement with the ring gear and coupling with the output shaft via the projected portion. As a result, the bearing may not sufficiently reduce inclination of the sun gear, and consequently, the gear efficiency of the rotary actuator may be decreased. Recently, it is desirable to reduce the size and weight of a rotary actuator. In addition, it is also desirable to enhance the gear efficiency of a rotary actuator.